1. The Field of the Invention
This invention is in the field of compositions and methods for bleaching teeth. More particularly, the invention is in the field of compositions and methods for accelerated bleaching activity using a two-part bleaching system. A first part includes a stable concentrated dental bleaching agent at low pH and a second part includes a dry particulate base mixed with a gelling agent.
2. The Relevant Technology
In the last decade there has been a proliferation of compositions and methods for bleaching teeth. Compositions for both in-office or at-home use typically include a peroxide bleaching agent, such as carbamide peroxide or aqueous hydrogen peroxide. Carbamide peroxide is a complex of urea and molecular hydrogen peroxide. On the other hand, hydrogen peroxide by itself only exists in aqueous form and is generally unstable except at relatively low pH (e.g., 2).
When formulating an appropriate bleaching composition there is inherently a trade-off between stability and reactivity. It is desirable for a bleaching composition to remain stable between the time when it is manufactured and when it is used to bleach a person""s teeth. However, once placed on a person""s teeth it is desirable for the composition to quickly break down and release active oxygen radicals capable of bleaching teeth. One way to obtain long-term stability is to keep the concentration of active bleaching agent relatively low. This, however, has the negative drawback of providing only minimal to modest bleaching activity.
Dental bleaching compositions that include higher concentrations of dental bleaching agent must generally be kept refrigerated to ensure long-term stability and/or kept at relatively low pH (e.g., 2 or less) where refrigeration cannot be guaranteed. In the case where it is desired to mass-produce a highly concentrated dental bleaching composition and then distribute it to many different locations, it is generally impossible to prevent at least a portion of such composition from becoming overheated at some point during shipping and handling. Unless such compositions are maintained at a low pH, their potency can be compromised if they are exposed to excessive heat during shipping. Accordingly, it has heretofore been difficult to manufacture and then distribute bleaching compositions at higher pH (e.g., neutral pH) that include high concentrations of bleaching agent while avoiding at least partial breakdown of the bleaching agent.
Since highly acidic compositions can etch or otherwise damage teeth if maintained in contact with the teeth for more than a few minutes, it is typically desirable to neutralize an acidic bleaching composition prior to application to a patient""s teeth. In the case where the composition to be applied to the patient""s teeth is intended to have a generally aqueous, non-viscous consistency, neutralization of an acidic aqueous hydrogen peroxide solution by means of a base is straightforward. One simple neutralization method is to add aqueous sodium hydroxide to the aqueous hydrogen peroxide until the pH is raised to the desired level. Depending on the concentration of the hydrogen peroxide and the resulting pH, raising the pH can also have the desired benefit of at least partially destabilizing the hydrogen peroxide so as to accelerate the release of oxygen radicals responsible for bleaching teeth.
Besides sodium hydroxide, there are a variety of bases or buffers that have been used to neutralize the acidity of stable hydrogen peroxide compositions. U.S. Pat. Nos. 5,713,738 and 5,645,428 to Yarborough, for example, discloses the use of sodium carbonate, sodium bicarbonate, calcium carbonate, ammonium hydroxide, potassium hydroxide, or calcium hydroxide to raise the pH of aqueous bleaching compositions so as to accelerate bleaching activity. For purposes of disclosing bases for use in neutralizing acidic hydrogen peroxide compositions, the foregoing patents are incorporated herein by reference.
One of the challenges in manufacturing two-part bleaching systems having an acidic bleaching component and a strong basic component, which are mixed together by the dental practitioner, is the difficulty in obtaining stable gels. Gels are often desirable over runny compositions because they adhere much better to the patient""s teeth compared to non-viscous liquids and stay in place rather than running off the teeth and onto surrounding soft oral tissues. Gels having a certain level of stickiness and viscosity are also desirable when using custom-fitting dental trays known in the art and exemplified by those disclosed in U.S. Pat. No. 5,376,006. For purposes of disclosing custom dental trays for bleaching teeth, the foregoing patent is incorporated herein by reference.
The ability to obtain a stable gel is particularly difficult where it is desired to use a polymeric tackifying agent such as carboxypolymethylene. Carboxypolymethylene is unable to maintain its desired stickiness and gelling capabilities over time when mixed with either concentrated acidic aqueous hydrogen peroxide solutions or strongly basic solutions used to neutralize the acidic hydrogen peroxide solution. Where it is desired to mass produce highly concentrated yet stable hydrogen peroxide bleaching compositions, it has heretofore been necessary to entirely replace carboxypolymethylene and like polymers with non-polymeric thickeners such as fumed silica in order to obtain stable gels.
In view of the foregoing, it would be an advancement in the art to provide improved two-part dental bleaching systems and methods of manufacturing such systems in order to yield a mixed composition that includes a polymeric thickening agent.
Such compositions and methods for providing multi-part dental bleaching systems yield a final mixed composition having a relatively high concentration of active hydrogen peroxide and a polymeric thickening agent are disclosed and claimed herein.
The present invention relates to dental bleaching systems and methods for manufacturing and using such bleaching systems to bleach teeth. More particularly, the invention encompasses multi-part dental bleaching systems that comprise a first part comprising a concentrated and stable acidic dental bleaching composition and a second part comprising a particulate base mixed together with a polymeric thickening agent as a neutralizing composition. Upon mixing the components together, the particulate base mixes with and raises the pH of the initially acidic bleaching composition, while the polymeric thickening agent yields a final gelled composition having desired stickiness, viscosity and other gel properties.
The multi-part system both preserves bleaching potency and ultimate stickiness or other gel properties of the final mixed composition during transport and subsequent storage. In other words, even after transport and storage, upon mixing the parts together a bleaching gel composition having a desired bleaching potency and gel properties can be formed. This allows the multi-part composition to be manufactured and then shipped and stored as desired prior to mixing the parts together without any significant loss in bleaching potency and destruction of ultimate gel properties. The invention also encompasses methods of manufacturing the inventive multi-part dental bleaching systems, as well as methods for bleaching a person""s teeth using such systems.
A first part or composition of the inventive dental bleaching system includes a dental bleaching agent. There are a variety of dental bleaching agents capable of releasing oxygen radicals. The most commonly used dental bleaching agents are based on hydrogen peroxide, including aqueous hydrogen peroxide or complexes of hydrogen peroxide, such as urea peroxide or a perborate (e.g., sodium perborate monohydrate). Sodium perborate monohydrate is a complex of sodium borate and molecular hydrogen peroxide. In one embodiment, the dental bleaching agent will advantageously comprise a concentrated aqueous solution of hydrogen peroxide (e.g., about 20-90% hydrogen peroxide, preferably about 30-60% hydrogen peroxide).
The pH of the first part or composition (i.e., the part that includes the dental bleaching agent) will typically be on the acidic side of the pH scale, generally less than about 5. In order to maintain the stability of the first part or component, particularly in the case where constant refrigeration cannot be ensured, it may be desirable to adjust the pH to below about 4, preferably to below about 3, and more preferably to a pH between about 1-2. The naturally low pH of aqueous hydrogen peroxide solutions can further be lowered by adding one or more acids in order to further increase stability (e.g., citric acid, which can also act as an ion scavenger).
The first part or composition may include optional additives in addition to the dental bleaching agent such as ion scavengers (e.g. EDTA or citric acid) or other stabilizing agents, thickening agents that are stable in the presence of hydrogen peroxide and/or low pH (e.g., fumed silica or gums), liquid polyols (e.g. PEG), and light-energy absorbing agents (e.g. carotene). Highly concentrated hydrogen peroxide bleaching compositions containing a variety of optional additives and adjuvents are set forth in U.S. Pat. No. 5,858,332 to Jensen et al. For purposes of disclosing highly concentrated dental bleaching compositions, as well as adjuvents and additives that may be included within such compositions, the foregoing patent is incorporated herein by reference.
A second part or composition of the inventive bleaching systems according to the invention includes a strong base in particulate form mixed with a polymeric thickening agent in a manner so as to preserve the ability of the thickening agent to impart desired thickening and tackifying properties when mixed with the bleaching composition to form the mixed composition. The neutralizing composition may be in many desired forms, e.g., a gel, a mixture of dry powders or particulates, or a paste. Examples of suitable bases include oxides, hydroxides, and carbonates of alkali and alkaline earth metals (e.g., finely ground potassium hydroxide or calcium carbonate). Examples of suitable polymeric thickening agents include carboxypolymethylene (which broadly includes carboxypolymethylene at any pH and at any degree of neutralization), PEMULEN (a proprietary composition available from B. F. Goodrich), and compositional or chemical equivalents thereof. The second part may optionally include additional components such as ion scavengers, other thickening agents, liquid polyols, and light-energy absorbing agents.
In order to prevent base-catalyzed destruction of the polymeric thickening agent, it is typically advantageous for the second part or composition to be substantially water free, preferably essentially anhydrous, since water is believed to hydrolyze or otherwise affect the stability of the polymeric thickening agent when mixed with a strong base. Because the second part is essentially water free, it does not technically have a pH. However, due to the inclusion of a strong base, the pH of the second part is hypothetically about 11-14 if mixed with sufficient water to produce an actual pH reading.
Upon mixing the first and second parts together, typically by the dental practitioner who will administer the bleaching composition to a patient""s teeth, the neutralizing component raises the pH of the bleaching agent so as to yield a mixed composition having a pH between about 4 and 11, more preferably between about 5 and 9, and most preferably between about 6 and 8. After mixing the multiple parts together, the mixed composition may either be used immediately by the dental practitioner for bleaching or else stored for later use, typically under refrigeration. Depending on the concentration of the dental bleaching agent, and/or the pH of the resulting mixed composition, and/or whether the mixed composition includes other components that affect the stability (e.g., stabilizing or destabilizing agents), the dental practitioner can optimize or adjust the stability of the mixed dental bleaching composition to suit the particular needs of the dental practitioner.
The relative stability or instability of the mixed dental bleaching composition depends, at least in part, on the pH, the temperature, and the relative concentration of the bleaching agent in the mixed composition. In general, the higher the pH and warmer the temperature, the less stable will be the bleaching agent at a given concentration. Accordingly, where it is desired to accelerate bleaching activity, the dental practitioner may wish to further raise the pH of the mixed composition in order to trigger the release of oxygen radicals, which are believed to be responsible for bleaching teeth. On the other hand, where it is desired to form a batch of mixed bleaching composition that can be stored in refrigeration for later use, it may be desirable to keep the pH at a level that does not trigger the release of oxygen radicals for a given concentration of bleaching agent.
The polymeric thickening agent within the neutralizing composition imparts a desired stickiness, tackiness, viscosity or other gel or thickening properties to the final mixed composition. Maintaining the neutralizing composition in substantially moisture free (e.g., less than about 1% unbound water) helps prevent the polymeric thickening agent from prematurely breaking down or hydrolyzing prior to formation of the final mixed composition.
The mixed composition will typically be used by dental practitioners during in-office procedures. This is particularly true for mixed compositions having relatively high concentrations of dental bleaching agent (e.g., about 20-90% available hydrogen peroxide), which are typically not suitable for at-home use. Nevertheless, the compositions of the present invention are not limited to any particular use or application and broadly include any two- or multi-part dental bleaching system that includes a bleaching agent in one part and a strong base dispersed within a stable gel in another part. Thus, home use products that incorporate the inventive multi-part bleaching systems disclosed herein are certainly within the scope of the invention.